(1) Field of the Invention
The present invention relates to a hydraulic drive device for a hydraulic excavator that has a front attachment connected to a swing body in a vertically rotatable manner and including an arm and a bucket.
(2) Description of the Related Art
Japanese Patent No. 3183815 discloses a hydraulic drive device which is provided in a hydraulic excavator having a front attachment including a boom, an arm, a bucket, a boom cylinder, an arm cylinder, and a bucket cylinder, and in which a boom directional control valve and a second arm directional control valve are connected in parallel tandem with a first hydraulic pump, and a first arm directional control valve is connected to a second hydraulic pump, the hydraulic drive device including an auxiliary switching valve that restricts the rate of flow supplied to the second arm directional control valve, following an arm roll-in operation (crowd operation).
In this hydraulic drive device according to the related art, the auxiliary switching valve is provided to prevent a situation where, during combined operation of arm roll-in and boom raise, the load pressure on the arm becomes lower than the load pressure on the boom, with the result that pressure oil in the first hydraulic pump is supplied only to the second arm directional control valve and is not supplied to the boom directional control valve. That is, during such a combined operation of arm roll-in and boom raise, the auxiliary switching valve is actuated to constrict the supply passage to the second arm directional control valve and raise the upstream pressure of the second arm directional control valve, thus securing the flow of pressure oil to the boom directional control valve, and pressure oil discharged from the second hydraulic pump is supplied to the first arm directional control valve, thereby performing a desired combined operation of arm roll-in and boom raise.